CD4+ T cells are major regulatory cells of the immune response. When stimulated by appropriate antigen-presenting cells (APC), naive CD4+ T cells mature into effector cells secreting cytokines that may regulate the growth and differentiation of a variety of cell types. A number of receptor-ligand pairs play key roles in the interaction between a T cell and an APC. The CD4 coreceptor binds to conserved sequences in the MHC protein, and together with the antigen-specific TCR/CD3 complex, initiates intracellular signaling. Additional nonantigen-specific molecules, such as the beta1 and beta2 integrins, facilitate conjugate formation and augment initial T cell signal transduction. For complete differentiation, however, current evidence suggests that naive T cells additionally require signals through costimulatory receptors, such as CD28. The complex, multivalent nature of the T cell-APC interaction makes it difficult to assign an adhesion or signal transducing role to any one accessory or costimulatory receptor. For this reason, we have employed purified, immobilized APC proteins, so that T cell responses may be examined in isolation. Using this approach, we have shown that mature CD4+ T cells undergo TCR-activated adhesion to many accessory ligands, including ICAM-1, fibronectin and vitronectin. Surprisingly, CD4-dependent binding to class Il proteins requires coengagement of the TCR and CD4 with the same class Il molecule. Since we had previously demonstrated that TCR engagement on mature CD8+ T cells induced cell adhesion to nonantigenic class I proteins, these findings suggest that coreceptor adhesion to MHC proteins is differentially regulated on CD4+ and CD8+ T cells. In this application, we propose to extend these studies by defining the intracellular processes that control stable T cell adhesion to antigenic class II proteins. In addition, we will employ transgenic mice expressing antigen-specific alpha-betaTCR, together with combinations of isolated peptide/class II complexes, ICAM-1, B7-1 and B7-2 to define more precisely the signal transduction pathways necessary for costimulation. The results should thus identify the accessory and costimulatory signals necessary for activation of naive CD4+ T cells, and their differentiation into effector cells.